This invention relates to pollution control and particularly to a system for processing flue gas containing sulfur dioxide and flyash. More particularly, this invention relates to an integrated system for flue gas processing incorporating spray scrubbing apparatus utilizing alkali slurries for sulfur dioxide removal and wet electrostatic precipitation apparatus for flyash removal.
Spray scrubbers in which a lime or limestone slurry is recycled through an absorption tower for contact with the flue gas to be cleaned are well known. The sulfur dioxide in the flue gas is dissolved in the droplets of the sprayed slurry and reacts with the alkali ions therein to produce an insoluble solid precipitate. For example, in U.S. Pat. No. 3,907,526, assigned to the assignee of this application, the disclosure which is incorporated herein by reference, there is disclosed a high velocity spray scrubber in which a plurality of vertically spaced, downwardly directed nozzles within the scrubber spray slurry droplets into the scrubber space while the gas to be scrubbed moves upward relative to the downwardly moving droplets of scrubber slurry.
In the spray scrubbing tower disclosed in U.S. Pat. No. 3,907,526, a wash tray is disposed between the upper end of the internal scrubbing space and the outlet of the scrubber, over which a layer of water flows for removing entrained droplets and particulates from the gas while an upwardly directed spray head is disposed beneath the wash tray to spray water onto the underside of the wash tray to prevent the formation of mineral deposits. The purpose of the wash tray is to remove entrained droplets of scrubber liquor and dust particles from the scrubbed gas. Disposed above the wash tray in the scrubber is a conventional mist eliminator for removing entrained droplets of spray liquor prior to exiting of the scrubbed gas from the scrubber.
It is also known to utilize the flyash present in the flue gas to be processed as a source of alkali by collecting the flyash, by use of either an electrostatic precipitator or a filter bag house or a venturi scrubber, as the flue gas leaves the boiler, removing alkali therefrom and using the alkali in place of lime or limestone in the scrubbing slurry. Such a process is disclosed in copending U.S. Patent Application Ser. No. 018,291, filed Mar. 6, 1979, assigned to the assignee of this application, the disclosure of which is incorporated herein by reference.
In the removal of particles from gas streams by electrostatic precipitation, the particle containing gas stream flows between grounded collection surfaces and corona discharge electrodes which produce ions in an electrostatic field causing the particles to be charged and attracted to the collection surface. In wet electrostatic precipitation, a liquid stream flows over the collection electrode to wash the particles therefrom.
In U.S. Pat. No. 1,329,818 there is disclosed a wet electrostatic precipitator in which furnace gases containing dust are contacted with water or solution to saturate them with water vapor. The saturated gases pass to an electrical precipitator wherein the temperature of the gases is reduced sufficiently to cause condensation of some of the water vapor to form a stream of condensed liquid on collecting electrodes. The walls of the collecting electrodes are maintained cooler than the gasses passing therethrough, by circulation of external air assisted by blower means or by other suitable cooling means, to cause condensation of moisture thereon sufficient to form a continuous stream of film to continually wash away collected materials.
In copending U.S. Patent Application Ser. No. 033,811, filed Apr. 27, 1979 now abandoned, assigned to the assignee of this application, the disclosure of which is incorporated herein by reference, there is disclosed a method and apparatus for self-cleaning wet electrostatic precipitation in which the collection electrode surfaces comprise tubes, incorporated between parallel tube sheets, which define channels through which a particle containing moisture-laden gas stream flows while a heat exchange medium, such as air, flows between the tube sheets, around and over the surface of the tubes to cause their collection surfaces to function as partial condensers. Water vapor present in the moisture laden gas stream condenses on the tube collection surfaces to continually flush them and prevent precipitated solids build-up.
It is stated in Application Ser. No. 033,811 now abandoned that such a system finds advantageous use together with conventional flue gas scrubbing apparatus in that down flowing condensate contacts up flowing gas and aids in the removal of gaseous contaminants such as sulfur dioxide in the flue gas. Moreover, the condensate has the quality of distilled water and is drained to equipment surfaces below to significantly reduce scaling tendencies. Since the condensed moisture is circulated through the system for reuse, the amount of water required for wet electrostatic precipitation is reduced and the amount of spray for the electrode surfaces can be reduced or eliminated entirely due to condensate flushing. Also, mist present in the gas downstream of the electrostatic precipitator decreases so that the amount of water lost by evaporation into the atmosphere is reduced.
Thus, there have been many and varied approaches to the processing of flue gas to remove gaseous contaminants such as sulfur dioxide and particulate matter such as flyash. However, these prior art attempts have focused largely on improving specific apparatus components or method steps which form but a part of the overall system and processing requirements for flue gas cleaning. Thus, there exists the need for additional and improved techniques which improve efficiency and economy in flue gas processing so that legislated emission requirements for steam generator operations can be met under varying operating conditions with greater efficiency in power utilization and the production of ecologically acceptable waste and/or other byproducts.
The present invention improves the efficiency and economy of known apparatus and methods for flue gas processing utilizing spray scrubbing and/or wet electrostatic precipitation and provides advantages not previously obtained.